31:1. On Shabbos (1) and on Yom Tov (Holidays) (2) it is forbidden to put on tefillin, because (3) Shabbos and Yom Tov themselves are called a sign, and if one would put on (4) another sign during these days, (5) it would belittle the "sign" provided by these holy days.

M.B. 6: Are also a "sign" - [The sign is] on Pesach the eating of matzah and on Succot sitting in the Succah. And the "some say" [i.e. the other opinion] reasons that since from the Torah, it is permitted to work on these days [even though the Rabbis prohibited some types of work - AB], there is no "sign".

M.B. 7: To wear them on Hol Ha-Moed - And the tefillin should be removed on Hol Ha-Moed before Hallel. And now, some people have made a custom of removing them after the Kedushah of Shemoneh Esrei [i.e. Amidah; of Shacharis], but they must yet be careful to pay attention and listen to the Chazzan's repetition of the [Shemoneh Esrei] prayer.